The present invention relates to fabrication of ultrasonic array transducers, particularly two-dimensional array ultrasonic transducers, in which individual piezoelectric elements of the array can be placed in desired positions to form an array without the limitations imposed by mechanical dicing.
Ultrasonic array transducers, employed for example in medical applications, rely on wave interference for their beam forming effects, and typically employ a plurality of individual transducer elements organized as either a one-dimensional (linear) array or a two-dimensional array. Ultrasound imaging is a non-invasive technique for obtaining image information about the structure of an object which is hidden from view, and has become widely used as a medical diagnostic tool. Ultrasound is also used for non-destructive testing and analysis in the industrial arts. Medical ultrasonic transducer arrays typically operate at a frequency within the range of one to ten MHz, although higher frequencies are certainly possible.
A two-dimensional phased array of ultrasonic transducer elements is often designed to obtain image data in two dimensions, without requiring movement of the array transducer.
Medical ultrasonic transducer arrays conventionally are fabricated from a block of piezoelectric material within which individual elements are defined and isolated from each other by sawing at least partially through the block of piezoelectric material, making a number of cuts with a dicing saw. In the case of a one-dimensional (linear) array, a series of dicing saw cuts are made parallel to each other. In the fabrication of a two-dimensional array, a second series of saw cuts is made at right angles to the first set of dicing saw cuts.
One of the limitations of this conventional process is that the positions of the array elements are limited by the nature of the dicing process. In addition, there is little control over the characteristics of the individual piezoelectric elements.
Relevant to the subject invention is a high density interconnect structure, also known as HDI, disclosed in Eichelberger et al. U.S. Pat. No. 4,783,695, and related patents. Very briefly, this high density interconnect structure employs a ceramic substrate which is made of alumina, for example, with a thickness between 25 and 100 mils. Using known ceramic processing techniques, metallic connection electrodes may be provided on the surface of the ceramic substrates, and electrical connections are made to these electrodes through either surface or buried conductors.
In the conventional HDI fabrication process at least one cavity is made in the ceramic substrate, and the various components, including semiconductor integrated circuit "chips", are placed in desired locations within the cavities and adhered with a thermoplastic adhesive layer.
A multi-layer interconnect overcoat structure is then built up to electrically interconnect the components into an actual functioning system. To begin the HDI overcoat structure, a polyimide dielectric film, which may be Kapton polyimide, about 0.0005 to 0.003 inch (12.5 to 75 microns) thick and available from E. I. du Pont de Nemours & Company, Wilmington, Del., is pretreated to promote adhesion and coated on one side with a thermoplastic such as Ultem.RTM. polyetherimide resin, available from General Electric Company, Pittsfield, Mass., and laminated across the top of the chips, other components and the substrate, with the Ultem resin serving as a thermoplastic adhesive to hold the Kapton film in place.
The actual as-placed locations of the various components and contact pads thereon are determined, and via holes are adaptively laser drilled in the Kapton film and Ultem adhesive layers in alignment with the contact pads on the electronic components. Exemplary laser drilling techniques are disclosed in Eichelberger et al. U.S. Pat. Nos. 4,714,516 and 4,894,115; and in Loughran et al. U.S. Pat. No. 4,764,485.
A metallization layer deposited over the Kapton film layer extends into the via holes to make electrical contact to the contact pads disposed thereunder. This metallization layer may be patterned to form individual conductors during its deposition, or it may be deposited as a continuous layer and then patterned using photoresist and etching. The photoresist is preferably exposed using a laser which is scanned relative to the substrate to provide an accurately aligned conductor pattern at the end of the process. Exemplary techniques for patterning the metallization layer are disclosed in Wojnarowski et al. U.S. Pat. Nos. 4,780,177 and 4,842,677; and in Eichelberger et al. U.S. Pat. No. 4,835,704 which relates to an "Adaptive Lithography System to Provide High Density Interconnect". Any misposition of the individual electronic components and their contact pads is compensated for by an adaptive laser lithography system as disclosed in U.S. Pat. No. 4,835,704.
Additional dielectric and metallization layers are provided as required in order to make all of the desired electrical connections among the chips.
HDI techniques have also been employed in connection with ultrasonic transducers. For example, Smith et al. U.S. Pat. No. 5,091,893, issued Feb. 25, 1992, entitled "Ultrasonic Array with a High Density of Electrical Connections" and assigned to the instant assignee discloses a piezoelectric ultrasonic array transducer having its individual elements connected to external electronics via a high density interconnect structure fabricated employing the HDI techniques briefly mentioned above. However, the individual piezoelectric elements in the Smith et al. ultrasonic array are formed employing the conventional dicing technique.